CHINO HILLS - Significant congestion, drilling, extensive road closures and disruption of traffic are all a possibility if the state Public Utility Commission approves constructing high-voltage power lines below streets and hills of this city.

That's the opinion of Southern California Edison - which would have to undertake that massive construction project - in a 43-page document submitted this week to the PUC.

The information was provided after the PUC ordered Edison to assess the cost and difficulty of undergrounding its power lines through Chino Hills instead of using above-ground power poles as planned.

Edison has previously gotten PUC approval to install expanded power poles through the city and has completed about one-third of the work. Objections by residents, the city and area legislators caused the project to be halted and further studies ordered.

The above-ground 500-kilovolt project is estimated to cost $166 million.

A proposal supported by opponents to run the power lines around the city and through the state park would cost from $424 million to $589 million.

In the most recent report, Edison estimates a cost of $300 million to $473 million to run a single line underground through the city and a double-circuit line from $703 million to more than $1 billion.

The below-ground option would require digging a 3.5-mile-long, 6-foot-wide trench across the city including across numerous city streets, according to the report.

That option would "involve some pretty extensive road closures and construction for putting that infrastructure under the street," said Charles Adamson, Edison's manager of major transmission projects.

Edison also said undergrounding a project with such voltage has never been undertaken in the United States. Undergrounding power lines up to 345 kilovolts is fairly common practice, and 500-kilovolt power lines on above-ground towers have been installed in Corona, Ontario and Fontana, Adamson said.

Earlier last month, Edison had turned over a 96-page document to the commission that detailed "feasibility, cost and timing" on 16 possible options for building the high-voltage Tehachapi Renewable Transmission Project through Chino Hills.

That report produced possible options through the state park, undergrounding the power lines, a different route through the city and/or state park, using the existing right-of-way but with shorter, more closely spaced towers, and ways to reduce the impact of larger towers.

This week's released report was ordered by the PUC after Chino Hills city officials told the agency Edison's report was incomplete as it did not include a single-circuit underground option.

The $2.1 billion power line project was approved in 2009 to carry 500-kilovolt wind-generated electricity from Kern County to the Los Angeles Basin.

Chino Hills officials have spent more than $2 million battling Edison in a lawsuit for two years insisting its right-of-way through the city is too narrow for the proposed 198-foot power towers.

City officials and the grassroots group Hope for the Hills have advocated building the project underground or through the neighboring Chino Hills State Park.

City officials and residents have also argued the presence of the transmission poles have reduced their home property value and pose a health and safety issue. They fear the larger 198-feet towers would fall onto their homes during an earthquake.

Edison officials said if there were a shift in the earth across where that infrastructure is, it could certainly create damage below ground.

Adamson said structures built above ground are very resilient to earthquakes, as well as wind, because of the way they are designed.

In this week's Edison report, it points out some impracticalities of undergrounded large power lines when repair is required. "The time of repair could range between five days and several months," the report said.

The undergrounding option also requires the construction of overground transition stations, used to cool the underground cable system, would have to be built along the route.

Chino Hills City Council member Ed Graham said Friday he understands the ramifications of the undergrounding option.

"If Edison moves as quickly to install the undergrounding option as they did to install the 200- foot towers throughout the city then we should be OK," he said. "After all, their transmission people do this for a living."

Graham said city officials and Edison are scheduled to have a mediation meeting the week of Feb. 13.

"The goal of the mediation is to see if we can come to an agreement on a route that is possible," he said.

"At any time during mediation, we could walk away or SCE could walk away, and if that happened then we would go into a full hearing with the CPUC again."